The function gpgme_sig_notation_add adds the notation data with
the name name and the value value to the context
ctx.

Subsequent signing operations will include this notation data, as well
as any other notation data that was added since the creation of the
context or the last gpgme_sig_notation_clear operation.

The arguments name and value must be NUL-terminated
strings in human-readable form. The flag
GPGME_SIG_NOTATION_HUMAN_READABLE is implied
(non-human-readable notation data is currently not supported). The
strings must be in UTF-8 encoding.

If name is NULL, then value should be a policy URL.

The function gpgme_sig_notation_add returns the error code
GPG_ERR_NO_ERROR if the notation data could be added
successfully, GPG_ERR_INV_VALUE if ctx is not a valid
pointer, or if name, value and flags are an invalid
combination. The function also passes through any errors that are
reported by the crypto engine support routines.